1. Field of the Invention
The present invention relates to the production of stencils for screen printing.
2. Related Background Art
The production of screen printing stencils is generally well known to those skilled in the art.
One method, referred to as the xe2x80x9cdirect methodxe2x80x9d of producing screen printing stencils involves the coating of a liquid light-sensitive emulsion directly onto a screen mesh. After drying, the entire screen is exposed to actinic light through a film positive held in contact with the coated mesh in a vacuum frame. The black portions of the positive do not allow light to penetrate to the emulsion which remains soft in those areas. In the areas which are exposed to light, the emulsion hardens and becomes insoluble, so that, after washing out with a suitable solvent, the unexposed areas allow ink to pass through onto a substrate surface during a subsequent printing process.
Another method, referred to as the xe2x80x9cdirect/indirect methodxe2x80x9d involves contacting a film, consisting of a pre-coated unsensitised emulsion on a base support, with the screen mesh by placing the screen on top of the flat film. A sensitised emulsion is then forced across the mesh from the opposite side, thus laminating the film to the screen and at the same time sensitising its emulsion. After drying, the base support is peeled off and the screen is then processed and used in the same way as in the direct method.
In the xe2x80x9cindirect methodxe2x80x9d a film base is pre-coated with a pre-sensitised emulsion. The film is exposed to actinic light through a positive held in contact with the coated film. After photochemical hardening of the exposed emulsion, the unexposed emulsion is washed away. The stencil produced is then mounted on the screen mesh and used for printing as described above for the direct method.
In the xe2x80x9ccapillary direct methodxe2x80x9d a pre-coated and pre-sensitised film base is adhered to one surface of the mesh by the capillary action of water applied to the opposite surface of the mesh. After drying, the film is peeled off and the screen then processed and used as described for the direct method.
In addition to the above methods, hand-cut stencils can be used. These are produced by cutting the required stencil design into an emulsion coating on a film base support. The cut areas are removed from the base before the film is applied to the mesh. The emulsion is then softened to cause it to adhere to the mesh. After drying, the base is peeled off. The screen is then ready for printing. This method is suitable only for simple work.
One problem generally associated with all the prior art methods is that many steps are necessary to produce the screen, thus making screen production time-consuming and labour-intensive.
Another problem is that normal lighting cannot be used throughout the screen production process in any of the methods except hand cutting. This is because the stencil materials are light-sensitive. In addition, it is necessary to provide a source of actinic (usually UV) light for exposing the stencil. This usually incurs a penalty of initial cost, space utilisation and ongoing maintenance costs.
Other methods of preparing printing screens are available. CA-A-2088400 (Gerber Scientific Products, Inc.) describes a method and apparatus in which a blocking composition is ejected directly onto the screen mesh surface in a pre-programmed manner in accordance with data representative of the desired image. The blocking composition directly occludes areas of the screen mesh to define the desired stencil pattern.
EP-A-0492351 (Gerber Scientific Products, Inc.) describes a method where an unexposed light-sensitive emulsion layer is applied to a screen mesh surface and a graphic is directly ink-jet printed on the emulsion layer by means of a printing mechanism to provide a mask through which the emulsion is exposed before the screen is further processed.
Both the above methods require the use of very specialised equipment (because of the need to handle large complete screens) which incurs a certain cost as well as imposing restrictions arising from the limitations of the equipment, in particular in terms of the size of screen and its resolution.
Ink-jet printers operate by ejecting ink onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink-jet printers can be used to produce a wide variety of printed materials, including text, graphics and images on a wide range of substrates. In many ink-jet printing systems, ink is printed directly onto the surface of the final receiving substrate. An ink-jet printing system where an image is printed on an intermediate image transfer surface and subsequently transferred to the final receiving substrate is disclosed in U.S. Pat. No. 4,538,156 (ATandT Teletype Corp.). Furthermore, U.S. Pat. No. 5,380,769 (Tektronix Inc.) describes reactive ink compositions containing at least two reactive components, a base ink component and a curing component, that are applied to a receiving substrate separately. The base ink component is preferably applied to the receiving substrate using ink-jet printing techniques and, upon exposure of the base ink component to the curing component, a durable, crosslinked ink is produced.
One object of the present invention is to make screen-printing stencil production less time-consuming and labour-intensive.
Another object is to allow normal lighting to be used throughout the stencil production process and to avoid both the problems of prior art stencil materials which are light-sensitive and also the need to provide a source of actinic (usually UV) light for exposing the stencil.
The present invention provides a method of producing a screen-printing stencil having open areas and blocked areas for respectively passage and blocking of a printing medium, the method comprising:
providing a receptor element comprising an optional support base and a stencil-forming layer which is capable of reacting with a chemical agent applied thereto to produce areas of lower solubility where application takes place and to leave higher solubility areas elsewhere, the areas of lower solubility being sufficiently adherent for attachment of the receptor element to a screen-printing screen after washing away of the areas of higher solubility from the receptor element;
applying the said chemical agent imagewise to the stencil-forming layer in areas corresponding to the blocked stencil areas;
washing away the stencil-forming layer in the higher solubility areas;
bringing the receptor element into contact with the screen in order to attach it to the screen by the adherency thereto of the areas of lower solubility; and
removing any remaining part of the receptor element in order to produce the screen-printing stencil.
In the method of the invention, the stencil is formed by chemical means without the need to use either special lighting conditions or actinic radiation.
Also, it is possible to carry out the method at reduced expenditure of time. and labour, compared with the known processes.
Advantageously, the chemical agent is applied dropwise to the stencil-forming layer.
Conveniently, the dropwise application is by use of an ink-jet device, for example an ink-jet printer or plotter. The device may have one or more ejection heads.
If desired, the chemical agent may be produced in situ by reaction between two or more precursor materials, separately applied to the stencil-forming layer, prior to contact with the stencil forming agent, at least one of which is applied in the said areas corresponding to the blocked areas of the stencil to be produced. This may conveniently be achieved by use of a plurality of drop-ejection heads.
When dropwise application is employed, the application is preferably controlled according to data encoding the desired pattern of blocked and open areas of the stencil to be produced. This control is conveniently by a computer, for example a personal computer. Thus, data representative of the desired output pattern can be input to a controller as pre-recorded digital signals which are used by the ejection head to deposit or not deposit the liquid containing the chemical agent as it scans the surface of the receptor element. The invention is not however restricted to dropwise application of the first chemical agent: other methods of application will achieve the same essential end, for example, the first chemical agent could be applied with a hand-held marker pen.
Preferably, the active component(s) of the chemical agent comprises one or more of:
boron salts, including boric acid, and Group I and Group II metal borates;
aldehydes, e.g. formaldehyde;
dialdehydes, e.g. glyoxal and glutaraldehyde, which may be activated by treatment with mineral acid;
isocyanates and their derivatives, including toluenediisocyanate;
carbodiimides and their derivatives, including pentahydroxy (tetradecanoate) dichromium and its derivatives; aziridine and its derivatives;
amines;
multifunctional silane compounds, including silicon tetraacetate;
N-methylol compounds, including dimethylolurea and methyloldimethylhydantoin; and
active vinyl compounds, including 1,3,5-triacryloyl-hexahydro-s-triazine,
optionally in a liquid solvent or carrier.
Advantageously, the active component(s) of the chemical agent constitutes from 0.5 to 100 wt. % of the chemical agent.
The invention also provides a pre-filled cartridge for a dropwise application device, for example an ink-jet printer or plotter, the cartridge containing one or more of the above chemical agents, optionally in a suitable liquid solvent or carrier.
For use in methods according to the invention, the invention also provides a receptor element comprising:
an optional support base;
a stencil-forming layer which is capable of reacting with a chemical agent applied thereto, to produce areas of lower solubility in a given solvent where application takes place and leave areas of higher solubility in the same solvent elsewhere, the areas of lower solubility being sufficiently adherent for attachment to a screen-printing screen to form thereon a stencil layer, after washing away of the areas of higher solubility.
The stencil-forming layer is preferably 5 to 20 xcexcm in thickness, more preferably 6 to 15 xcexcm.
Although not essential, the receptor element preferably has a support base which can be removed, preferably by peeling away, as a part of the receptor remaining after the receptor element has been applied to the screen.
The support base conveniently comprises polyethylene terrephthalate, polyethylene, polycarbonate, polyvinyl chloride, polystyrene or a coated paper, its thickness preferably being from 10 to 200 xcexcm.
It is desirable for there to be a release layer between the support base and the stencil-forming layer. Suitable release layer comprise one or more of: polyurethanes, polyamides, polyesters, nitrile rubbers, chloroprene rubbers, polyvinyl acetate and polyacrylates.
The release layer is preferably from 0.1 to 5 xcexcm in thickness, more preferably 0.5 to 1 xcexcm.
Advantageously the stencil-forming layer comprises two or more different polymeric substances.
Preferably, the stencil-forming layer comprises two or more sub-layers, each of a respective different substance or blend of two or more different substances.
Particularly suitable materials comprise one or more of the following polymers:
polyvinylalcohol and its derivatives;
polyvinyl acetate;
gelatin and its derivatives;
carboxylated polymers capable of becoming water soluble on addition of alkali, including carboxylated acrylics, ethylene-acrylic acid and styrene-acrylic acid copolymers;
water-soluble cellulose derivatives, including starch and
hydroxy propyl cellulose;
sulphonated polymers;
polyacrylamides;
epoxy resins; and
amino resins, including urea-formaldehyde and melamine-formaldehyde.
It is particularly preferred that the stencil-forming layer comprises a blend of a first grade of polyvinyl alcohol, having a first, higher degree of hydrolysis, and a second grade of polyvinyl alcohol, having a second, lower degree of hydrolysis.
When the stencil-forming layer comprises two or more sub-layers, the outermost sub-layer preferably comprises a blend of a first grade of polyvinyl alcohol, having a first, higher degree of hydrolysis, and a second grade of polyvinyl alcohol having a second, lower degree of hydrolysis. Preferably, the next outermost sub-layer comprises a blend of polyvinyl alcohol and polyvinyl acetate.
Advantageously, the first and second degrees of hydrolysis fall within the ranges 86% to 92% and 76% to 82%, respectively.
Preferably, the ratio by weight of the first to the second grade of polyvinyl alcohol falls within the range of from 1:9 to 9:1, more preferably in the range of from 1:3 to 3:1.
Advantageously, the number average molecular weight of the first grade of polyvinyl alcohol is lower than that of the second grade.